In the technology of chromatography, the separation and collection of fractions of chemical compounds vary in process, instruments, and production capability. However, each type of system uses at least a fluid, called a mobile phase, that is pumped into the head of a separation column. A sample containing one or more chemical compounds is carried by the mobile phase flowstream through the column. The media in a column delays certain compounds from exiting the outlet of the column according to different retention times. These separated compounds can be detected and graphed as a “peak” of the injected sample. If the same temperature, pressure, flowrate, and injection composition is maintained in the chromatography system, then repeated injections into the column of the same mixture can produce repeated peaks of the same compounds exiting the column. These eluted peaks contain purified samples that can be collected in a collection system.
The parameters and instruments of the chromatography system can be adjusted in order to optimize the speed, efficiency, and accuracy of analyte collection. An advancement of liquid chromatography (LC) is HPLC (High Performance Liquid Chromatography), which uses 20 mm to one inch diameter columns with flowrates optimized at 20 to 30 ml/min. While process and collection speeds are faster than LC, drawbacks to HPLC include high waste solvent production and slow effective process time for samples due to removal of solvent and water from collected sample fractions.
For many applications, an alternative separation technology called supercritical fluid chromatography (SFC) has advanced past other chromatography technologies. SFC uses highly compressible mobile phases, which typically employ carbon dioxide (CO2) as a principle component. In addition to CO2, the mobile phase frequently contains an organic solvent modifier, which adjusts the polarity of the mobile phase for optimum chromatographic performance. Since different components of a sample may require different levels of organic modifier to elute rapidly, a common technique is to continuously vary the mobile phase composition by linearly increasing the organic modifier content. This technique is called gradient elution.
SFC has been proven to have superior speed and resolving power compared to traditional HPLC for many applications. This results from the dramatically improved diffusion rates of solutes in SFC mobile phases compared to HPLC mobile phases. Separations have been accomplished as much as an order of magnitude faster using SFC instruments compared to HPLC instruments using the same chromatographic column. A key factor to optimizing SFC separations is the ability to independently control flow, density and composition of the mobile phase over the course of the separation. SFC is finding significant advantages in the separation of enantiomers and is supplanting normal-phase HPLC for performing chiral separations.
Conventional LC columns, during manufacture, are constructed with end fittings such as a frit that act to retain the packing material inside the column. The packing media material is loaded to completely fill the column and retained at both ends by the fittings that each include a frit element. The frit acts as both a seal to hold the packing media inside the column and as a radial distribution element to distribute the incoming fluid substantially evenly across the cross-section of the column. In some commercial columns, the frit is initially press-fit into the fitting prior to the fitting being placed on the end of the column. In the prior art, columns are often machine tooled at the ends, which causes stresses and weakening at the ends that can cause premature breakage.
Problems occur with chromatography columns that are caused by rapid and/or uncontrolled pressure loss occurring downstream of the outlet side of the column in a process system. If the mobile phase flowstream contains a gas liquefied under pressure, such as carbon dioxide, then rapid depressurization will cause the gas to rapidly evaporate and freeze and in the case of carbon dioxide freeze form dry ice. The pressure differential between the inlet at full pressure and outlet at atmospheric pressure in a column can be as high as 100 bar. Expanding gas throughout the column media bed builds tremendous pressure forces against the outlet of the column. Since frits are commonly metal that are fitted into a column with a plastic ring, the freezing of the metal and the freezing of the plastic ring will cause differential swelling between the frit and its fittings. Further, if pressure is instantly relieved in a column of packed media bed containing a flowstream of carbon dioxide, dry ice will form in the media bed itself and disturb the packing consistency of the bed. The combination of differential shrinking of the frit and fittings and disturbance in the bed caused by expanding mobile phase under pressure force towards the column outlet could cause loss of packed media past the frit.
The instant release of pressure from an expanding mobile phase in a column can also damage the frit itself. A frit that is instantly frozen by dry ice becomes brittle, and its channels normally used for distributing fluid flowstreams become clogged up with dry ice, which blocks all flow out of the column. With a clogged outlet to the column and the column media still under rising high pressure, the failure of the frit along with damage to column media can occur. In larger columns, the larger diameter frit is even more susceptible structural to deflection caused by clogging and pressure from the packed media. A larger frit has less structural support at its center. If the frit freezes and becomes brittle, it can crack or break upon deflection from the upstream pressure forces attempting to escape the column. Further, the pressure differential between the ends of the column can cause an impulse force directly onto the frit whether or not the channels in the frit freeze, causing potential failure of the frit and its fittings.